This invention relates to heat exchangers and, more particularly, to a design which reduces thermally induced loads in the tube support plates and the tube bundle wrapper of a nuclear steam generator.
A typical steam generator for a pressurized water nuclear reactor comprises a vertically oriented heavy wall rolled cylindrical shell, a thin wall round wrapper portion disposed within the shell portion and spaced therefrom, a plurality of U-shaped tubes disposed within the wrapper portion so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the bent sections thereof, a plurality of generally parallel tube support members longitudinally spaced within the wrapper portion and having a plurality of holes through which the tubes pass, and a plurality of individual radial support means angularly spaced about the wrapper portion and associated with the shell and wrapper for radially constraining the tube support members.
Each radial support means includes a jacking block, which is welded into the wrapper, and jacking screws which are threaded through holes in the jacking block to align and support the wrapper within the shell, creating an annular space between the shell and wrapper. The jacking screws are then welded to the jacking block. The tube support members or plates are then positioned within brackets which provide longitudinal support. Wedges are driven between the jacking blocks and the side edges of the support plates while in the unheated shop condition and then welded, creating a structural load path between the support plates and the shell. The load path transmits dynamic loads, which may occur during steam generator handling and shipping or from seismic events, from the tube bundle to the shell, thereby inhibiting yielding of any of the tube bundle or tube support members.
Nuclear steam generators are subject to a variety of corrosion mechanisms. At the intersections of the tube support members with the tubes, the corrodents present in the secondary side water tend to concentrate to the detriment of both the tubes and the tube support members. Use of a support member with improved corrosion resistance will also reduce tube corrosion. However, materials with higher corrosion resistance will generally contain higher amounts of nickel and chromium than carbon steel, resulting in higher coefficients of thermal expansion. As the steam generator is heated to operating temperature, the greater expansion of the corrosion resistant tube support members versus that of the carbon steel wrapper results in undesirable and usually unacceptable stress levels in the tube support members and in the wrapper. Such stresses could lead to local yielding.
Therefore, what is needed is a steam generator which can accommodate the difference in thermal expansion between the tube support members and the wrapper without inducing unacceptably high stresses in the steam generator components while maintaining a rigid radial structural load path between the tube support members and the shell.